Internet Engineering Task Force (IETF) P. Hoffman
Request for Comments: 6605 VPN Consortium
Category: Standards Track W.C.A. Wijngaards
ISSN: 2070-1721 NLnet Labs
April 2012
Elliptic Curve Digital Signature Algorithm (DSA) for DNSSEC
Abstract
This document describes how to specify Elliptic Curve Digital
Signature Algorithm (DSA) keys and signatures in DNS Security
(DNSSEC). It lists curves of different sizes and uses the SHA-2
family of hashes for signatures.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6605.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Hoffman & Wijngaards Standards Track [Page 1]

RFC 6605 ECDSA for DNSSEC April 20121. Introduction
DNSSEC, which is broadly defined in RFCs 4033, 4034, and 4035
([RFC4033], [RFC4034], and [RFC4035]), uses cryptographic keys and
digital signatures to provide authentication of DNS data. Currently,
the most popular signature algorithm is RSA with SHA-1, using keys
that are 1024 or 2048 bits long.
This document defines the DNSKEY and RRSIG resource records (RRs) of
two new signing algorithms: ECDSA (Elliptic Curve DSA) with curve
P-256 and SHA-256, and ECDSA with curve P-384 and SHA-384. (A
description of ECDSA can be found in [FIPS-186-3].) This document
also defines the DS RR for the SHA-384 one-way hash algorithm; the
associated DS RR for SHA-256 is already defined in RFC 4509
[RFC4509]. [RFC6090] provides a good introduction to implementing
ECDSA.
Current estimates are that ECDSA with curve P-256 has an approximate
equivalent strength to RSA with 3072-bit keys. Using ECDSA with
curve P-256 in DNSSEC has some advantages and disadvantages relative
to using RSA with SHA-256 and with 3072-bit keys. ECDSA keys are
much shorter than RSA keys; at this size, the difference is 256
versus 3072 bits. Similarly, ECDSA signatures are much shorter than
RSA signatures. This is relevant because DNSSEC stores and transmits
both keys and signatures.
In the two signing algorithms defined in this document, the size of
the key for the elliptic curve is matched with the size of the output
of the hash algorithm. This design is based on the widespread belief
that the equivalent strength of P-256 and P-384 is half the length of
the key, and also that the equivalent strength of SHA-256 and SHA-384
is half the length of the key. Using matched strengths prevents an
attacker from choosing the weaker half of a signature algorithm. For
example, in a signature that uses RSA with 2048-bit keys and SHA-256,
the signing portion is significantly weaker than the hash portion,
whereas the two algorithms here are balanced.
Signing with ECDSA is significantly faster than with RSA (over 20
times in some implementations). However, validating RSA signatures
is significantly faster than validating ECDSA signatures (about 5
times faster in some implementations).
Some of the material in this document is copied liberally from RFC6460 [RFC6460].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Hoffman & Wijngaards Standards Track [Page 2]

RFC 6605 ECDSA for DNSSEC April 20122. SHA-384 DS Records
SHA-384 is defined in FIPS 180-3 [FIPS-180-3] and RFC 6234 [RFC6234],
and is similar to SHA-256 in many ways. The implementation of SHA-
384 in DNSSEC follows the implementation of SHA-256 as specified in
RFC 4509 except that the underlying algorithm is SHA-384, the digest
value is 48 bytes long, and the digest type code is 4.
3. ECDSA Parameters
Verifying ECDSA signatures requires agreement between the signer and
the verifier of the parameters used. FIPS 186-3 [FIPS-186-3] lists
some NIST-recommended elliptic curves. (Other documents give more
detail on ECDSA than is given in FIPS 186-3.) These are the same
curves listed in RFC 5114 [RFC5114]. The curves used in this
document are:
FIPS 186-3 RFC 5114
------------------------------------------------------------------
P-256 (Section D.1.2.3) 256-bit Random ECP Group (Section 2.6)
P-384 (Section D.1.2.4) 384-bit Random ECP Group (Section 2.7)
4. DNSKEY and RRSIG Resource Records for ECDSA
ECDSA public keys consist of a single value, called "Q" in FIPS
186-3. In DNSSEC keys, Q is a simple bit string that represents the
uncompressed form of a curve point, "x | y".
The ECDSA signature is the combination of two non-negative integers,
called "r" and "s" in FIPS 186-3. The two integers, each of which is
formatted as a simple octet string, are combined into a single longer
octet string for DNSSEC as the concatenation "r | s". (Conversion of
the integers to bit strings is described in Section C.2 of FIPS
186-3.) For P-256, each integer MUST be encoded as 32 octets; for
P-384, each integer MUST be encoded as 48 octets.
The algorithm numbers associated with the DNSKEY and RRSIG resource
records are fully defined in the IANA Considerations section. They
are:
o DNSKEY and RRSIG RRs signifying ECDSA with the P-256 curve and
SHA-256 use the algorithm number 13.
o DNSKEY and RRSIG RRs signifying ECDSA with the P-384 curve and
SHA-384 use the algorithm number 14.
Hoffman & Wijngaards Standards Track [Page 3]

RFC 6605 ECDSA for DNSSEC April 2012
Conformant implementations that create records to be put into the DNS
MUST implement signing and verification for both of the above
algorithms. Conformant DNSSEC verifiers MUST implement verification
for both of the above algorithms.
5. Support for NSEC3 Denial of ExistenceRFC 5155 [RFC5155] defines new algorithm identifiers for existing
signing algorithms to indicate that zones signed with these algorithm
identifiers can use NSEC3 as well as NSEC records to provide denial
of existence. That mechanism was chosen to protect implementations
predating RFC 5155 from encountering resource records they could not
know about. This document does not define such algorithm aliases.
A DNSSEC validator that implements the signing algorithms defined in
this document MUST be able to validate negative answers in the form
of both NSEC and NSEC3 with hash algorithm 1, as defined in RFC 5155.
An authoritative server that does not implement NSEC3 MAY still serve
zones that use the signing algorithms defined in this document with
NSEC denial of existence.
6. Examples
The following are some examples of ECDSA keys and signatures in DNS
format.
6.1. P-256 Example
Private-key-format: v1.2
Algorithm: 13 (ECDSAP256SHA256)
PrivateKey: GU6SnQ/Ou+xC5RumuIUIuJZteXT2z0O/ok1s38Et6mQ=
example.net. 3600 IN DNSKEY 257 3 13 (
GojIhhXUN/u4v54ZQqGSnyhWJwaubCvTmeexv7bR6edb
krSqQpF64cYbcB7wNcP+e+MAnLr+Wi9xMWyQLc8NAA== )
example.net. 3600 IN DS 55648 13 2 (
b4c8c1fe2e7477127b27115656ad6256f424625bf5c1
e2770ce6d6e37df61d17 )
www.example.net. 3600 IN A 192.0.2.1
www.example.net. 3600 IN RRSIG A 13 3 3600 (
20100909100439 20100812100439 55648 example.net.
qx6wLYqmh+l9oCKTN6qIc+bw6ya+KJ8oMz0YP107epXA
yGmt+3SNruPFKG7tZoLBLlUzGGus7ZwmwWep666VCw== )
Hoffman & Wijngaards Standards Track [Page 4]

RFC 6605 ECDSA for DNSSEC April 20127. IANA Considerations
This document updates the IANA registry for digest types in DS
records, currently called "Delegation Signer (DS) Resource Record
(RR) Type Digest Algorithms". The following entry has been added:
Value 4
Digest Type SHA-384
Status OPTIONAL
This document updates the IANA registry "Domain Name System Security
(DNSSEC) Algorithm Numbers". The following two entries have been
added to the registry:
Number 13
Description ECDSA Curve P-256 with SHA-256
Mnemonic ECDSAP256SHA256
Zone Signing Y
Trans. Sec. *
Reference This document
Number 14
Description ECDSA Curve P-384 with SHA-384
Mnemonic ECDSAP384SHA384
Zone Signing Y
Trans. Sec. *
Reference This document
* There has been no determination of standardization of the
use of this algorithm with Transaction Security.
8. Security Considerations
The cryptographic work factor of ECDSA with curve P-256 or P-384 is
generally considered to be equivalent to half the size of the key, or
128 bits and 192 bits, respectively. Such an assessment could, of
course, change in the future if new attacks that work better than the
ones known today are found.
The security considerations listed in RFC 4509 apply here as well.
Hoffman & Wijngaards Standards Track [Page 6]